In recent years, services for distributing digitalized image and audio content via broadcast waves, such as satellite or ground waves, or networks are in practical use, and highly efficient encoding is thus necessary for efficiently recording and transmitting the information of images and sounds that have tremendous amount of information. As content has become finely-resolved and more diverse, the number of images to be encoded and the amount of data have become more increased. Thus, more highly efficient encoding of image signals based on possible future content services is required.
As highly efficient encoding of images, a method represented by MPEG2 and MPEG4-AVC (Advanced Video Coding) is used, which is for compressing the amount of information by means of a correlation between pixels that are spatially adjacent to each other in the same frame of a moving image signal and a correlation between frames or fields that are temporally adjacent to each other.
In MPEG4-AVC (ISO/IEC 14496-10 Advanced Video Coding), high encoding efficiency is achieved by dividing an image into a plurality of two-dimensional blocks, generating a prediction signal by means of a correlation in the same frame or between frames on a block-by-block basis, and then encoding differential information relative to the prediction signal. A prediction process in MPEG4-AVC in which a correlation in the same frame is used is referred to as intra prediction, and a prediction image of an encoding target block is generated by means of a decoding image of an already encoded portion that is adjacent to the target block, as shown in FIG. 11. In intra prediction, a prediction mode with least errors relative to an encoding target block is selected based on a prediction image generated in a plurality (nine kinds when the prediction is performed in units of 4×4 pixel blocks) of prediction modes on the assumption that adjacent decoding images have a high correlation in a certain direction as shown in FIG. 11 so as to perform encoding along with prediction mode information.
Intra prediction is a prediction process in which only a correlation with an adjacent area is used. Thus, when there is only low correlation at a border with an encoding target block, an advantage of the prediction is reduced.
In an image signal, there exists a signal that has image components of a similar shape, pattern, background, etc., of an object even at a position that is apart from a target block. Patent document No. 1 presents a method for performing a prediction process by means of an image correlation at a position apart from a target block. More specifically, an error between an already-encoded decoding image, at a position obtained by moving a target block by an displacement amount (hereinafter, referred to as a “displacement vector”) within a screen as shown in FIG. 12, and the encoding target block is calculated, and a reference image referred to by a displacement vector with the least error is set to be a prediction image so as to encode the prediction image along with the displacement vector.
High encoding efficiency can be achieved for intra prediction when a prediction image with high correlation that outweighs an encoding amount for transmitting a displacement vector can be generated. However, sufficient prediction efficiency cannot be realized when the amount of transmitting a displacement vector amount is large.
In order to reduce an encoding amount required for the displacement vector, patent document No. 2 presents a method for identifying the displacement vector without transmitting the displacement vector. In patent document No. 2, an error between an already encoded decoding image, at a position obtained by moving an already-encoded decoding image adjacent to a target block by the displacement vector while using the already-encoded decoding image as a template, and the already-encoded decoding image adjacent to the target block is calculated, and a displacement vector with the least error is used as a displacement vector of the encoding target block so as to set a reference image referred to by the displacement vector to be a prediction image. In this method, a displacement vector can be calculated, without receiving the displacement vector, by detecting the displacement vector using an already encoded decoding image in decoding, as in the case of encoding. Thus, there is no increase in the encoding amount caused due to additional information.    [Patent document No. 1] JP 2005-159947    [Patent document No. 2] JP 2007-043651